1. Field of the Invention
This invention relates to novel nitromalonate polyesters, to methods for preparing said nitromalonate polyesters, and to solid propellants which comprise a binder, an oxidizer, a plasticizer and, optionally, a metallic fuel wherein the binder is a nitromalonate polyester.
2. Description of the Prior Art
Solid propellants are commonly made by preparing a mixture of a finely divided organic or inorganic oxidizing agent, a metallic fuel, a liquid polymer binder, a curing agent for the polymer, a plasticizer and minor amounts of various modifying ingredients, introducing the resulting mixture into a motor casing and curing the mixture. The cured polymer acts both as a fuel for reaction with the oxidizing agent and as a binder to provide the propellant with the desired physical properties.
One problem with solid propellants has been that, when a plasticizer was employed in the propellant composition, the propellant frequently was subject to plasticizer syneresis and crystallization. Attempts were made to overcome this problem and at least partial success was met with propellants which employed a combination of polyethylene glycol (PEG) and polycaprolactone (PCP) as the binder and a plasticizer which was a mixture of trimethylolethane trinitrate (TMETN) and butanetriol trinitrate (BTTN). This combination of PEG/PCP binder and TMETN/BTTN plasticizer was found to produce minimum smoke propellants which had excellent strain capability under low temperature storage and cycling conditions. These propellants were also less prone to crystallization of either the polymer binder or plasticizer. They were further found to be resistant to plasticizer exudation or synersis at intermediate low temperatures such as 0 to -20.degree. F. The combination of plasticizers was found to be a very important factor in the prevention of crystallization at low temperatures. In fact, certain plasticizers or combinations of plasticizers, such as TMETN and 1/1 mixtures of TMETN and BTTN, crystallize after low temperature storage and cause severe loss in propellant strain capability to a level of less than 1-2%.
While the combination of 1 part TMETN and 2 parts BTTN does provide acceptable propellant properties, due to the very high cost of BTTN it would be highly desirable to develop a propellant which eliminates or decreases the level of BTTN needed to prevent plasticizer crystallization while still maintaining the desirable physical properties and burn characteristics of the propellant.
Demands for higher performance, longer range minimum smoke rocket missions are a recurring theme by all military services as second generation rocket motor requirements approach or exceed the limits imposed by present state-of-the-art materials. Though these demands exist for strategic and strap-on launch applications, the most stringent requirements are mandated by tactical application. The tactical environment imposes high and low temperature requirements on the propellants which can severely limit the utilization of otherwise attractive propellant ingredients. Problems encountered include limitations of shelflife imposed by marginal high-temperature stability and propellant cracking and/or plasticizer syneresis. All of these problems have intensified as higher levels of nitrate ester plasticizers have been used to approach the performance requirements of current systems. The inherent thermal instability of nitrate esters, volatility, tendency toward phase separation and crystallization at low temperature impose important trade-off considerations which can limit their utilization. On the other hand, propellant processing and mechanical property requirements also place upper limits on the level of nitramines which can be formulated.
State-of-the-art polymer ingredients, with the exception of nitrocellulose, are non-energetic materials. Because minimum smoke propellants, in general, are under-oxidized since the nitramine "oxidizers" are actually monopropellants, the inert polymer causes a steep decline in specific impulse as the plasticizer/polymer ratio is decreased. If sufficiently stable energetic or oxidizing groups can be incorporated into the polymer without otherwise degrading the binder properties, then the propellant performance and specific impulse can be significantly increased.
Another important consideration in designing high energy polymers is the composition of the exhaust gases after combustion. Higher oxygen content binders reduce the content of hydrogen and carbon monoxide in the exhaust gases. Reducing these fuel-rich gases is necessary if the afterburning of the exhaust plume is to be effectively minimized. This consideration, along with smoke reduction, is important in designing minimum signature rocket motors.
Historically, the original high-energy polymeric material was nitrocellulose. Although nitrocellulose was used early for non-energetic applications such as motion picture film and billiard balls, its principle use has been in the explosives and munitions industry. Depending on the degree of nitration of the cellulose, the cellulose and the materials with it is mixed, nitrocellulose finds applications in smokeless powder and double-base propellants. The nitrate ester linkage (RONO.sub.2) present in nitrocellulose provides a very high and energetic oxygen content; however, like the nitrate ester plasticizers such as nitroglycerine, nitrocellulose has marginal thermal stability for tactical environmental scenarios.
In recent years, other energetic functional groups have been incorporated into polymers as well as plasticizer and oxidizer materials. In some cases, such as the extensive research and development effort into organic nitrogen-fluorine chemistry in the 1960's, no materials are currently being considered for high energy applications. Factors which have limited the application of N-F compounds include cost, chemical stability, and sensitivity.
It will be noted that nearly all of the high-energy polymers currently under investigation are hydroxyl functional. These materials, in conjunction with an isocyanate cure agent, are favored in minimum smoke formulations which contain nitrate esters because of excellent compatibility. Other potential functional groups such as carboxylic acid and thiols, which have been used in composite propellant formulations, are incompatible with nitrate esters. This requirement and the ready availability of a wide variety of hydroxyl-terminated polymers has led to their favored position for use in minimum smoke binders.
Thus, it would be highly desirable to provide a polymeric binder for solid propellants which is compatible with the other propellant ingredients, provides acceptable propellant physical properties, prevents or minimizes plasticizer syneresis and crystallization and has a high-energy potential which will contribute to the propellant's specific impulse.